This invention relates to a drum unit, and more particularly may e applied to that used for a video tape recorder, for example.
In conventional video tape recorders (VTRs), there is used a tracking technique in which by the use of control signals (hereinafter referred to as CTL pulses) recorded in control (CTL) tracks in a magnetic tape, tracking of helical recording tracks which correspond to the CTL pulses are performed.
As shown in FIG. 1, in a magnetic tape 1 of a VTR, a helical recording track TR.sub.O is obliquely formed by a helical scan of the rotary head, and a CTL track TR.sub.CTL is formed at the lower edge of the magnetic tape 1.
In view of spaces for traveling and loading of the magnetic tape 1, the CTL head for recording CTL pulses in and reproducing them from such CTL track TR.sub.CTL is practically arranged at a remote position outside the drum unit.
A desired matching in phase to the helical recording track TR.sub.O is carried out in a signal processing system, and it is thereby possible to record a CTL pulse which corresponds to the helical recording track TR.sub.O.
There is a VTR in which an improvement is achieved in recording density as a whole by reducing helical recording tracks TR.sub.O on the magnetic tape 1 in track width and by thinning the magnetic tape 1.
In such a case, the magnetic tape 1 makes an extension or a contraction due to a change in environment and a change according to lapse of time. This causes an error to occur in tracking of helical recording tracks TR.sub.1 which correspond to CTL pulses due to deformation of the recording pattern.
The length of the helical recording tracks TR.sub.O is determined according to the diameter and the wrapping angle of the drum unit. For example, in the component digital VTR according to the SMPTE D-1 format, the length is selected to 170 mm.
When the magnetic tape recorded in such a manner changes in recording pattern due to a change according to lapse of time or the like, as shown in FIG. 2, tracking may be made so as to provide a maximum reproduction level at the central portion of each of the helical recording tracks TR.sub.O for reproduction to minimize degradation of reproduced signals S.sub.RF.
Even if the magnetic tape 1 extends or contracts, it is accordingly possible to efficiently obtain reproduced signals S.sub.RF by recording CTL pulses, which correspond to the helical recording tracks TR.sub.O, at central portions of the helical recording tracks TR.sub.O.
For this purpose, there is a VTR in which a conventional drum unit is provided with a CTL head for recording CTL pulses on and reproducing them from CTL tracks TR.sub.CTL, and thereby CTL pulses are recorded in and reproduced from positions corresponding to central portions of the helical recording tracks TR.sub.O.
More specifically, in such a VTR, a magnetic tape 1 is as shown in FIG. 3 wrapped around a drum unit 4 at a predetermined wrapping angle by inclining guides 2 and 3, and helical recording tracks TR.sub.O on the magnetic tape 1 are recorded and reproduced by a rotary head 6 disposed in a gap 5 between an upper drum 4A and a lower drum 4B.
Here, the magnetic tape 1 is guided by an upper edge guide 4C, formed in the upper drum 4A, and a lower edge guide 4D which is a step formed in the lower drum 4B. Thereby the magnetic tape 1 is capable of sliding on the circumferential surface of the drum unit 4.
Furthermore, the drum unit 4 is provided at a central portion of the lower edge guide 4D with a circular hold 4E into which a control head 7 is fitted and embedded. The control head 7 is formed in a cylindrical shape by joining mask members to a core member having a coil and wiring members attached to it and having a recording and reproducing gap 7A. CTL pulses are recorded at and reproduced from positions which correspond to central portions of the helical recording tracks TR.sub.O.
Since the CTL head 7 is formed in the cylindrical shape, a magnetic powder or the like which is produced by contacting a sliding surface of the magnetic tape 1 is previously prevented from being deposited. The fitting of the CTL head 7 into the circular hole 4E of the drum unit 4 makes it detachable, and hence improves maintenance.
Between the magnetic tape sliding surface of the drum unit 4 and the magnetic surface of the magnetic tape 1, there is formed an air film layer, which enables smooth traveling of the magnetic tape 1 by the so-called fluid, lubrication function.
This air film layer air film layer is also generated between the CTL head 7 and the magnetic tape 1, where recording and reproducing errors are liable to occur due to the so-called spacing loss in a case where in recording or reproduction, the CTL head 7 is not brought into contact with the magnetic tape 1.
For this reason, in a VTR using the drum unit 4, an air film is, as shown in FIG. 4A, made thin by making the tape tension as high as possible. At the same time, a magnetic tape contact surface (hereinafter referred to as a head surface) of the CTL head 7 in which contact surface a recording and reproducing gap 7A is formed is designed in the shape of a curved surface having a radius of curvature in the magnetic sliding direction, the radius of curvature R.sub.H being slightly smaller than a drum radius R.sub.D. This construction enhances the urging force of the magnetic tape 1 against the head surface of the CTL head 7, and thereby the production of the spacing loss is effectively avoided.
However, when in the drum unit 4 with such a construction, the tape tension in the magnetic tape 1 is enlarged as described, the urging force of the rotary head 6 against the magnetic tape 1 becomes larger. This causes the rotary head 6 to be liable to easily wear, and increases sliding noises of the magnetic tape 1, so that S/N ratio deteriorates.
Moreover, there is a problem in that a load is applied to the loading guides of the magnetic tape 1 themselves according to the tape tension, resulting in that the magnetic tape 1 is not smoothly guided to thereby provide a damage to the magnetic tape 1. This also results in that the life of the guides of the loading mechanism becomes short by the wearing thereof.
In addition to these, there is a problem in that unerased parts are liable to be produced in a case where erosure and writing of CTL pulses are performed only by the recording and reproducing gap 7A. In an extreme case, illegal formatting may take place, thereby making reproduction impossible.
This problem can be overcome by arranging a CTL head for erosure only in addition to the CTL head 7 as recording and reproducing head, but such a solution makes the construction and the fabrication step of the head rather complicated. As shown in FIG. 4B, use is thus made of a CTL head 8 with the so-called double gap construction in which an erosure gap 8B is provided in addition to the recording and reproducing gap 8A.
In the case as this where the recording and reproducing gap 8A and the erosure gap 8B are formed in the CTL head 8, it is necessary to wrap the magnetic tape 1 over the two gaps 8A and 8B with spacing kept as small as possible.
However, merely equating the radius of curvature R.sub.H of the CTL head 8 in the magnetic tape sliding direction with that of the CTL head 7 of FIG. 4A makes it difficult to secure a sufficient wrap of the magnetic tape 1 and produces unnecessary spacing.
For this reason, it is considered to slightly project the CTL head 8 from the magnetic tape sliding surface of the drum unit 4 but this raises a problem in that the rotary head 6 cannot sufficiently contact the magnetic tape 1.
As specifically shown in FIG. 5, in a case where the rotary head 6 projects 20 micrometers, for instance, from the magnetic tape sliding surface of the drum unit 4, the CTL head 8 also is projected 20 micrometers. The air gap 9A of the magnetic tape 1 between the rotary head 6 and the CTL head 8 becomes markedly thicker than an air film layer 9B.
As a result, a tent T0 is not sufficiently formed around the rotary head 6, so that a spacing loss is produced. As shown in FIG. 6, this results in that the level of a reproduced envelope waveform ENV0 obtained from the rotary head 6 drops at a reproduction position P.sub.CTL which corresponds to the CTL head 8. The recording and reproduction operation is hence not correctly carried out, and this solution is still not sufficient.
Moreover, the projection of the CTL head 8 by 20 micrometers from the magnetic tape sliding surface of the drum unit 4 produces a problem such that the magnetic tape 1 is raised upwardly, so that helical recording tracks TR.sub.O are deteriorated in linearity.